Electron-diffracting device



Feb. 19, 1963 .1. B. LE POOLE E'IAL 3,078,368

ELECTRON-DIF'FRACTING DEVICE Filed NOV. 4, 1960 INVENTOR JAN B. LE POOLE JOHANNES KRAMER United States Patent ()flice 3,078,368 Patented Feb. 19, 1963 3,078,368 ELECTRON-DIFFRACTING DEVICE Jan Bart Le Poole and Johannes Kramer, Delft, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Nov. 4, 1960, Ser. No. 67,376 Claims priority, application Netherlands Dec. 10, 1959 3 Claims. (Cl. 250-495) This invention relates to electron-difiracting devices compirsing an electron gun, an electron-focusing system and an electron-optical lens. Such a device permits the production of diffraction images of an object, placed in the beam of rays, on a fluorescent screen or a photographic plate. The use of an electron-optical lens is not strictly necessary, but focusing the electrons on the collecting screen results in the definition and also the intensity of the image being increased.

A possibility of adjusting the desired sharpness of focusing consists in observing the light spot produced on the fluorescent screen at the area Where the electrons which are not deflected in the object strike the screen. However, this method is unsatisfactory, since the intensity of the electron beam which is necessary to produce a visible diffraction image considerably exceeds the value for which the luminescence of the fluorescent material is saturated. It is known that measuring the diameter of such a light spot is impossible.

A conventional method of determining the diameter of the spot consists in that the spot is caused to describe a line on a fluorescent screen covered with a phosphorescent material and, subsequently, the width of the line is measured. For this purpose, the screen is moved transversely to the axis of the electron beam. This operation must be repeated several times if, by varying the refractive power of the lens, it is endeavoured to find the adjustment for which the width of the line is a minimum. Such a method of determining the optimum electron-optical adjustment is complicated and time-consuming and does not satisfy particularly high requirements. An object of the invention is to obviate this disadvantage. According to the invention, in an electron-diffraeting device of the said kind there are arranged as close as possible to the collecting screen and in the axis of the electron beam, a target and opposite the surface of the target at the edge of the collecting screen, a fluorescent screen and also between the target and the fluorescent screen a fine-meshed metal- Lic grid, an image of which is reproduced on the fluorescent screen by the deflected electrons. This image can be seen through a window provided in the wall of the device. The angle made by the surface of the target and the axis of the ditiracting device is chosen to be such that a rotation-symmetrical section of the beam yields a rotation-symmetrical lack in definition of the shadow image.

Due to the reflection, a small proportion of the electrons striking the target is scattered towards the fluorescent screen. The target must be made of heavy metal in order to avoid excessive penetration of the electrons and preferably consists of platinum or tungsten.

The image of the metal grid on the fluorescent screen serves to adjust the refractive power of the electronic lens to the value which is optimum for projecting the electronic source on the collecting screen. The definition of the image of the grid depends upon the size of the electron spot on the target and hence upon the accuracy of focusing the electron beam. The target and the metal grid are preferably arranged so as to obtain a greatly enlarged image of the metal grid on the fluorescent screen.

In order that the invention may be readily carried into eifect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing showing one embodiment of an electron-difl racting device according thereto.

The device comprises an electron-emissive cathode 1 and an electrode 2, which serves as a control electrode for adjusting the intensity of the electron-current. Then follows an electrode 3 having a small aperture 4 for separating the marginal rays. This assembly of electrodes, in which a considerable positive potential is applied between the cathode 1 and the electrode 3, concentrates the electrons to form a beam. A condenser lens 5 serves to reproduce an image of the narrowest area of the beam on the collecting screen. An object holder 6 contains the object of which the diffraction image is produced. The holder 6 is secured to a carrier 7 and can be removed for changing the object through an aperture in the wall of the device which is closed by means of a cover 8. In order to obtain a sharp diffraction image, the lens 5 must be adjusted with high accuracy. With proper choice of the energization, all of the image points are adjusted sharply, as well as the beam spot on the collecting screen pro-. duced by the electrons which emerge from the object without being deflected. In order to facilitate the adjustment of the lens 5, the said electrons, which together form an epipolar ray, are collected by a target 9 the surface of which reflects electrons. If in total 5% of such electrons are reflected, a current density is obtained sufliciently great to cause bright luminescence of a fluorescent screen 10 positioned along the edge of a collecting screen 11. A fine-meshed metal grid 12 is arranged between the target 9 and the fluorescent screen 10. The shadow image of the grid 12 is produced on the fluorescent screen 10 by the reflected electrons.

A wall 13 of the device has a window 14 through which the image produced on the fluorescent screen 10 can be observed. With the aid of this image, it is possible to adjust the energization of the electronic lens 5 so that the image acquires its maximum definition.

The device described easily permits of testing the definition of the image at any desired moment. The target 9 may be so small that the production of the diffraction image on the collecting screen is not hindered by it. In the embodiment described, the target 9 and the metal grid 12 are secured to an arm 15 which can pivot about a centre of rotation 16 provided at the edge of the collecting screen 11, so that the target 9 may be removed from the beam by turning the arm 15 about the point 16 and moving it into a position 17 as indicated in broken lines.

Platinum or tungsten may be used as the metal of the target.

What is claimed is:

1. An electron-diflracting device comprising an electron gun for projecting an electron beam along a given axis and an electron-optical system for producing a diffraction image of an object, placed in the beam of rays,

- on a collecting screen for the electrons, and as close as possible to the collecting screen and intersecting the axis of the electron beam 2. target having an electron-reflecting surface and, opposite the target, a fluorescent screen and also between the target and the fluorescent screen a finemeshed metal grid, an image of which is reproduced on the fluorescent surface by the reflected electrons.

2. A device as claimed in claim 1, in which the target and the metal grid are arranged so as to obtain an enlarged image of the metal grid on the fluorescent screen. 10

3. A device as claimed in claim 2, in which the target 4 and the metal grid are secured to an arm having a centre of rotation at the edge of the collecting screen and which can occupy a position in which the target and the grid are located outside the cone of rays for the diffraction image.

References Cited in the file of this patent UNITED STATES PATENTS 2,273,235 Von Ardenne Feb. 17, 1942 2,360,871 Griswald Oct. 24, 1944 2,606,292 Columbe Aug. 5, 1952 

1. AN ELECTRON-DIFFRACTING DEVICE COMPRISING AN ELECTRON GUN FOR PROJECTING AN ELECTRON BEAM ALONG A GIVEN AXIS AND AN ELECTRON-OPTICAL SYSTEM FOR PRODUCING A DIFFRACTION IMAGE OF AN OBJECT, PLACED IN THE BEAM OF RAYS, ON A COLLECTING SCREEN FOR THE ELECTRONS, AND AS CLOSE AS POSSIBLE TO THE COLLECTING SCREEN AND INTERSECTING THE AXIS OF THE ELECTRON BEAM A TARGET HAVING AN ELECTRON-REFLECTING SURFACE AND, OPPOSITE THE TARGET, A FLUORESCENT SCREEN AND ALSO BETWEEN THE TARGET AND THE FLUORESCENT SCREEN A FINEMESHED METAL GRID, AN IMAGE OF WHICH IS REPRODUCED ON THE FLUORESCENT SURFACE BY THE REFLECTED ELECTRONS. 